The present invention relates to a GaN semiconductor and/or its related material's device (AlGaInN semiconductor device).
A fact has been known that the GaN type semiconductor device can be employed as, for example, a blue light emitting device. A light emitting device of the foregoing type incorporates a substrate usually made of sapphire.
The sapphire substrate has the following problems which must be solved. That is, since the sapphire substrate is transparent, light emitted from the light emitting device and required to be extracted from the upper surface of the device undesirably penetrates the sapphire substrate which is formed in the opposite surface of the device. Therefore, light emitted by the light emitting device cannot effectively be used.
Moreover, the sapphire substrate is still an expensive substrate.
In addition, since the sapphire substrate is an insulating material, the electrodes must be formed on same side of the substrate. Thus, a portion of the semiconductor layer must be etched. It leads to a fact that the bonding process is doubled. Since both of n and p electrodes are formed on one side, reduction in the size of the device has been limited. What is worse, a problem of charge up arises.
Although use of a Si (silicon) substrate in place of the sapphire substrate may be considered, growth of a high quality AlGaInN semiconductor layer on the Si substrate is very difficult. One of causes is the difference in the coefficient of thermal expansion between Si and the GaN type semiconductor. In contrast with the linear coefficient of thermal expansion of Si which is 4.7.times.10.sup.-6 /K, the linear coefficient of thermal expansion of CaN is 5.59.times.10.sup.-6 /K which is larger than the former value. Therefore, if the GaN semiconductor layer on Si is cooled after it is grown, the Si substrate is expanded. Thus, the device is deformed such that the semiconductor layer portion of the GaN is compressed. There is apprehension that tensile stress is generated and thus a crack is formed. Even if no crack is formed, lattices are distorted. Therefore, a required function of the AlGaInN type semiconductor device cannot be exhibited.